It was 'my turn in the barrel' last night. My FT-5 happily finished the last of four 4 hour prints, parked itself, shut down the part fan, heat bed and hot end, and then promptly went poof! I posted over on FB but that will disappear by this evening.

Upon inspection, the only part destroyed that I can see is the MOV on the input. That little fella takes a beating evidently. It just crumbled when I went to move it away from the cross line capacitor block to which it was melted....

I've ordered another PS plus some Chinese MOV'S that will eventually show up. Then I'll see if I can resurrect this one and , perhaps, use it to power just the heated bed.

I upgraded my heated bed a while back to the FT black bed. It came with a 4 amp fuse to replace the 3 amp one in the switch block; however, as you will notice in the photo, the PS has an internal 3 amp fuse so how much protecting the 4 amp fuse does is in question.

The MOV is there, I believe, to protect the supply from in-rush current and spikes in the voltage as it charges the capacitors. It dissipates this excess as heat so, over time, I'm guessing they just get brittle and fall apart as this one did.

I thought someone might find the pictures useful in the future so this is the place to park them.

Sorry to hear that happened.Actually, that isn't an MOV, it is a negative temperature coefficient thermistor.It is indeed present to lower the inrush current, and if nothing else is damaged you might be able to resurrect the PSU if you replace it.It is supposed to run hot, but that one clearly ran a bit too hot. They run about $3 on eBay (NTC 5D-15).The internal fuse is supposed to be a 6.3 amp device as indicated on the board legend.So an external 4 amp fuse would protect the internal fuse (though obviously not anything else).By any chance are you still running the as-shipped cabinet fan and vent holes?If so you may want to consider upgrading them to improve internal cooling.

Hi Ron,I've been running it with the front panel off for the last couple of months as I got too lazy to keep taking it off and putting it on as I added the external control for the heated bed. Pure laziness.

You're right. It is a varistor, not an MOV as some suppliers label it. I took the board out of the case a little while ago and noticed that it's in line with the neutral lead, which wouldn't be right for an MOV. It would be ACROSS the power and neutral.

Ummm not exactly, it's not a varistor either.It is an NTC thermistor (or negative-temperature-coefficient resistor as you prefer).Varistors and MOVs are voltage-dependent-resistors (an MOV is just a Metal Oxide Varistor).

Thermistors vary their internal resistance based upon temperature.

They are two completely different animals.

When the component that failed is cold it has a relatively high resistance.I didn't find an accessible spec sheet on it, but I suspect from the marking that it may have around 15 ohms resistance at room temperature.When you turn on the AC it resistively limits the charge up current spike for the large input filter capacitors.They will appear as a virtual dead short for an AC cycle or two and the spike could otherwise be upwards of a hundred amps or even more.A current spike like that could easily pop the fuse(s) and destroy the power switch, and doesn't do anything any good in general.As you place the PSU under an output current load the device self-heats from current loading.As it self-heats its resistance goes down, typically with a sharp "knee" in the resistance-temperature curve.At some point it reaches an equilibrium point where the current versus resistance becomes stable.But to get there it might have to get quite hot (you wouldn't want to touch it).Do that enough times and it will eventually crack and/or go up in smoke.It is a cheapskate method of limiting the inrush current, but it can be quite effective.

You just drew a winning card in the stock PSU lottery this time around.

It almost sounds like these should be replaceable parts every so often. As brittle as mine was, I assume they can only take so many heat cycles. Makes me want to pull my 2020 i3 supply out and replace its THERMISTOR when I get them. Thanks for the lesson, Mr. Wizard. Maybe you're too young to remember him. Hell, I remember black and white Howdy Doodie.

Ah yes, I remember Mr. Wizard (Don Herbert) quite well.I wrote him a letter once and actually received a letter in reply.I wanted to know what a chemical he used in an episode was.He wouldn't tell me (I figured it out later anyway).It was probably written by a secretary or helper, but he signed it (or at least it looked like it).I may even still have that letter somewhere.I also watched Howdy Doodie, Sky King, Space Angel, and others of that era.But I also watched Sunrise Semester on the Texas Educational Network and loved it. We didn't get our first color television until I was in college.

Hello....the internal fuse is supposed to be a 6.3 amp device as indicated on the board legend.So an external 4 amp fuse would protect the internal fuse.By any chance are you still running the as-shipped cabinet fan and vent holes?